1. Field of the Invention
The present invention relates to a clathrate compound, a thermoelectric material using thereof and a method for producing the thermoelectric material.
2. Description of Related Art
A thermoelectric element using the Seebeck effect is capable of converting heat energy into electric energy. Such a thermoelectric element is now receiving attention as one of energy-saving technologies from the viewpoint of environmental issues because the element can convert exhaust heat from industrial/consumer processes and mobile units into an available electric power by utilizing the property.
A dimensionless figure of merit ZT of a thermoelectric element using the Seebeck effect is expressed by the following equation (1).ZT=S2T/ρκ  (1)where S, ρ, κ and T are the Seebeck coefficient, electrical resistivity, thermal conductivity and temperature, respectively.
It is apparent by the equation (1) that what is important to improve capacity of a thermoelectric element is to make the Seebeck coefficient of an element be large, an electrical resistivity be small and a thermal conductivity be small.
Conventionally, thermoelectric elements made of bismuth telluride, silicon germanium-based materials, lead telluride and the like are known as high-performance thermoelectric materials.
Each of these conventional thermoelectric elements has a problem to be solved. For example, the bismuth telluride have a high ZT value at a room temperature; however, the ZT value becomes small drastically at beyond 100 degrees C., and thus it is difficult to use the material as a thermoelectric material at 200-800 degrees C. which is necessary for generation of electric power from exhaust heat. On the other hand, the bismuth telluride and the lead telluride contain lead and/or tellurium that will cause environmental load.
Therefore, new thermoelectric materials which have sufficient thermoelectric property, cause less environmental load and have a light weight are requested to be developed. A clathrate compound is receiving attention as one of such new thermoelectric materials.
A composition and a method for producing a clathrate compound composed of Ba, Ga, Al and Si are already disclosed. Patent Document 1 discloses a single crystal and a producing method of Ba8(Al, Ga)xSi46-x in which silicon atoms are substituted by Al atoms or Ga atoms by the number of x (10.8≦x≦12.2) per unit lattice. Patent Document 2 discloses that a P-type Ba—Al—Si clathrate compound has a ZT of 1.01 at 700 degrees K.
However, these clathrate compounds have following problems.
The technique disclosed in Patent Document 1 (Tokukai 2004-67425A) does not disclose the ZT value and there is a concern for the property. Patent Document 2 (JP Patent No. 4413323, paragraph 0048, for example) discloses the p-type compound; however, a ZT value of an n-type compound is not disclosed and that may be a concern for the property.
A single-phase Si clathrate may be the best material for obtaining a high dimensionless figure of merit ZT. However, there is a problem that the single-phase Si clathrate for obtaining a high dimensionless figure of merit ZT generates a crack easily.